1. Field of the Invention
The invention relates to a sampler for taking a sample from raw engine or vehicle exhaust, or from a primary dilution system.
2. Background Art
The United States Environmental Protection Agency (EPA) defines engine emitted particulate matter (PM) as material collected by filtration of diluted exhaust with a filter temperature of not more than 52° C. A large number of dilution systems have been developed to meet this criterion.
The constant volume sampler (CVS) is a full flow dilution tunnel. The CVS takes all engine exhaust and dilutes the exhaust proportionally. The dilution process in the CVS is straightforward, and is very accurate for gaseous and PM measurement. However, the CVS is expensive, large in size, and not portable. As well, a specific CVS may only be used for a particular range of engine sizes.
Another approach to diluting engine exhaust is provided by the partial flow sampling system. The partial flow sampling system takes a small fraction of flow from the engine exhaust. In comparison to the CVS, the partial flow sampling system has advantages such as being relatively inexpensive, having a smaller size, as well as being useable for any size of engine, etc. Due to the nature of the dilution process, a complicated control system must be integrated for proportional control during transient testing conditions. Additional information, such as real-time engine exhaust flow rate, needs to be provided for dilution control during the transient testing conditions.
The conventional partial flow sampling system controls dilution air flow and total flow. The total flow is defined as the mixture of the dilution air and the sample flow. The difference of the total flow and dilution air flow is the sample flow rate. The following equations define the sample flow, the dilution ratio, and the sample ratio at time ti:
                              Q          sample_i                =                              Q                          total_              ⁢              i                                -                      Q            dilutionair_i                                                            Dr          i                =                              Q            total_i                                Q            sample_i                                                            r          i                =                                            Q              sample_i                                      Q              exhaust_i                                *          100                    where, Qsample—i is the sample flow rate into the partial flow sampling system at standard or reference conditions, Qtotal—i is the total flow rate at the standard or reference conditions, Qdilutionair—i is the dilution air flow rate at the standard or reference conditions, Dri is the dilution ratio, and ri is the sample ratio.
When the sample ratio r and the total flow Qtotal are constant in the partial flow sampling system, the partial flow sampling system simulates the constant volume sampler (CVS) by operating with proportional control. In this situation, more sample flow is taken when exhaust flow rate is higher, and less sample flow is taken when exhaust flow rate is lower. As well, the partial flow sampling system may be operated at constant dilution ratio when the system takes constant exhaust flow and the total flow remains constant.
In the conventional partial flow sampling system, the sample flow rate is obtained from the difference between the total flow and the dilution air flow. Accordingly, significant errors in the sample flow may be generated when the sample flow is small, in other words, when the dilution ratio is high or the sample ratio is low. In this way, small errors in total flow and dilution air flow may result in large errors in the sample flow. As a result, the accuracy of the dilution ratio or the sample ratio decreases when the dilution system runs under high dilution ratio or low sample ratio. The control on the dilution ratio or the sample ratio drifts. This eventually influences the accuracy of the PM measurement.
As discussed above, the accuracy of the sample flow rate (Qsample—i) strongly influences the accuracy of the dilution ratio and sample ratio. As a result, the PM number and mass measurement are influenced. Based on the working principle of the conventional partial flow sampling system, the conventional partial flow sampling system cannot avoid significant errors on the sample flow rate under some conditions.
For the foregoing reasons, there is a need for a sampler for engine exhaust dilution that provides accurate sample flow measurements over a wide range of dilution ratios and sample ratios, thereby providing more accurate results for PM number and mass measurements.